(1) Field of the Invention
This invention is related to the conversion of hydrocarbon streams using a catalytic reforming multizone process and more particularly to catalytic reforming of naphtha fractions over a first catalyst containing tin and a platinum group metal followed by contacting with a second catalyst containing a platinum group metal.
(2) General Background
The reforming of hydrocarbon naphtha streams is an important petroleum refining process employed to provide high octane hydrocarbon blending components for gasoline or chemical processing feedstocks.
Catalytic reforming of naphthas can be carried out through the use of several types of catalysts and in fixed or moving bed processes. Catalysts employing a platinum group metal as a hydrogenation component and rhenium as a promoter are often employed in reforming processes.
Within the last ten years many companies have promoted the use of catalysts which contain additional components to enhance the catalytic properties of reforming catalysts. One of these components which is used commercially is tin. Typically, tin is placed on an alumina support making up a reforming catalyst containing platinum and optionally rhenium.
It is known that a platinum-tin reforming catalyst generally gives a higher C.sub.5 + yield at constant conversion as measured by octane number than platinum-rhenium catalysts or catalyst containing just platinum. Furthermore, platinum-tin catalysts are more stable than platinum catalysts and less stable than platinum-rhenium catalysts.
In pilot plant tests using identical feedstocks and different catalysts, these results were confirmed. The platinum-tin catalyst did show increased C.sub.5 + yields when compared to a standard commercially available platinum-rhenium catalyst while the platinum-rhenium catalyst showed greater stability than the platinum-tin catalyst. In an experiment, which will be described in detail later, a mixed loading test was performed in which a platinum-tin catalyst was used in the initial two of three reaction zones of the pilot unit followed by a platinum catalyst in the last reaction zone. The selectivity exhibited by this mixture of catalysts surprisingly showed that benzene, toluene and xylene (BTX) yields with the mixed catalyst loading were greater than with either catalyst when tested independently.
I have found in the early part of the reaction train where dehydrocyclization is predominant, high yields of heavy aromatics are produced by the platinum-tin catalyst which possesses a higher paraffin dehydrocyclization selectivity. These heavy aromatics are dealkylated to BTX by the platinum or platinum-rhenium catalyst in the latter stages of the reaction train where the hydrocracking reaction is predominant. There is no need for the tin containing catalyst to be present in the latter zones of a multizone reforming process, since the predominant reaction taking place there is hydrocracking. I have also found that platinum-rhenium catalysts are more stable than platinum-tin catalysts which makes the former a better choice for use in the latter stages of a multizone reforming process where catalyst deactivation is typically greater.
An advantage, therefore, exists in a reforming process having at least two segregated catalyst zones where the first zone contains a first catalyst containing tin and at least one platinum group metal (e.g., tin and platinum). The second zone contains a second catalyst containing at least one platinum group metal (e.g., platinum, preferably platinum and rhenium) and preferably has an essential absence of tin.
This means the second catalyst should contain low amounts of tin, since the preferred second catalyst is platinum-rhenium which is more stable than a tin-containing catalyst in the latter stages of a reforming process. An essential absence of tin generally means concentrations of tin of less than about 0.1 weight percent of the catalyst and preferably less than about 0.05 weight percent. Tin can be present in minor amounts in the second catalyst through various sources, such as contamination in manufacture or contact with equipment, such as reactors or catalyst loading equipment, or from tin carry-over from upstream catalysts or equipment.
The improved BTX yields are of considerable economic importance, and furthermore, the BTX yield improvement is not at the expense of the C.sub.5 + yield which also increased. Thus, the advantages in improved quality of liquid product are not accompanied by a reduction in overall liquid product and, in cases where platinum-rhenium catalysts are used in the latter reaction stages, overall catalyst activity can be more easily obtained.
It can be seen that the application of this invention, therefore, leads to improved profitability of reforming operations in that liquid yields and especially the valuable BTX segment is increased. Further, since more active platinum-rhenium catalysts can be used in the latter stages of a multi-stage reforming process where improved catalyst stability results in higher octane numbers, this invention does not detract significantly from ability to meet expected future requirements for higher reformate octanes which will be required in many refineries.
Stone, U.S. Pat. No. 3,864,240 discloses a two-stage reforming process in which a fixed-bed comprises the first reaction zone and one or more moving beds comprise the second reaction zone in the process. The catalyst used in such a process can be a Group VIII noble metal combined with a halogen component placed on a porous carrier material which may contain various modifiers including rhenium and tin.
In U.S. Pat. No. 4,212,727, Antos, a single-stage reforming process is disclosed employing a commingled physical catalyst mixture of a first catalytic composite comprising palladium on a zeolite aluminosilicate carrier material and a second catalytic composite comprising alumina, platinum, and a platinum promoter including tin.
In U.S. Pat. No. 4,032,475, Knapik et al., a catalyst and process are disclosed for the reforming of hydrocarbons in which the catalyst system comprises a physical mixture of particles made up of platinum group metals, tin, halogen and cobalt mixed with dual-function catalysts of the prior art typically containing platinum and rhenium.
In European Pat. No. 153,891 issued Sept. 4, 1985, corresponding to U.S. Pat. No. 4,588,495 issued May 13, 1986, based on French Application No. 842926 filed Feb. 23, 1984, there is disclosed a reforming process giving high quality gasoline with good catalyst stability which employs a platinum-rhenium catalyst in the first bed of a multi-stage reaction process followed by one or more beds of a catalyst comprising platinum, and tin, thallium or indium. It should be noted that in this patent the teaching of a mixed catalyst system requires the platinum and tin composite be in the latter stages of the reforming process.
In U.S. Pat. No. 3,705,095, Dalson et al., a two-stage reforming process is disclosed comprising a naphthene dehydrogenation zone having a catalyst containing platinum and having an essential absence of rhenium followed by a paraffin dehydrocyclization zone having a catalyst obtaining platinum and rhenium.